The present invention relates to a centrifugal pump for pulp suspensions of lignocellulosic material. More particularly, the present invention relates to a pump where gas, such as air, can be separated from the pulp in the pump.
The presence of gas, which follows along with the pulp, causes problems during pumping by centrifugal pumps. The gas in the pulp forms into bubbles, which accumulate in front of the impeller and thereby deteriorate the capacity of the pump. This problem can be remedied by evacuating the gas from the space in front of the impeller, for example by means of vacuum pumps of the liquid ring type. The pumps are often a separate vacuum pump, the suction pipe of which is connected to the sealing space of the pulp pump. They can also be designed as a separate vacuum impeller in the form of a liquid ring impeller located on the shaft of the pulp pump so that a common drive for both impellers is obtained.
In those cases when a separate vacuum pump is used, the pulp pump becomes relatively complicated and expensive, because more components are required in the system. When a separate liquid ring impeller of a conventional type is placed on the same shaft as the pulp pump, the capacity of the liquid ring impeller is restricted for space reasons.
In accordance with the present invention, these and other problems in the prior art have been eliminated by the discovery of apparatus for pumping a pulp suspension including entrained gas comprising a housing including an inlet for the pulp suspension and an outlet for the pulp suspension, a rotary shaft, an impeller mounted for rotation on the rotary shaft within the housing, the impeller having a front face and a rear face and including a central hub, a wheel disk surrounding the central hub, a plurality of pump blades extending radially from the central hub along the wheel disk, the wheel disk including at least one opening for the discharge of the entrained gas therethrough, a vacuum pump disposed at the rear face of the impeller, the vacuum pump including a vacuum pump housing, a plurality of vacuum pump blades mounted for rotation within the vacuum pump housing, the vacuum pump housing including an outer wall eccentrically disposed with respect to the plurality of vacuum pump blades and a rear vacuum pump wall including a suction port for receiving the entrained gas, the vacuum pump housing being mounted adjacent to the central hub to provide a gap therebetween, an exhaust port for exhausting the exhaust gas therefrom, the vacuum pump housing being axially adjustably mounted whereby the gap between the central hub and the vacuum pump housing can be altered by axial adjustment of the vacuum pump housing, a gas passage disposed radially outside the outer wall of the vacuum pump housing for connecting the at least one opening in the wheel disk with the suction port, and a gas evacuation chamber connected to the exhaust port for evacuation of the entrained gas therefrom. In accordance with a preferred embodiment of the apparatus of the present invention, the apparatus includes a liquid inlet connected to the gas passage for the supply of liquid thereto.
In accordance with another embodiment of the apparatus of the present invention, the apparatus includes a passageway disposed outside the outer wall of the vacuum pump housing for connection of a separate vacuum pump to the gas passage.
The present invention suggests that these problems can be eliminated by the fact that the vacuum impeller is designed to be integrated into the impeller of the pulp pump. The construction is compact and simple, and the capacity of the vacuum pump can be easily controlled. Moreover, the arrangement is formed so that it can easily be combined with a separate vacuum pump so that both pumps can be used in parallel. Alternatively, the separate vacuum pump can be connected at an operation drop-out of the integrated vacuum pump according to the present invention in order thereby to ensure the accessibility for the centrifugal pump.